# Doubt from rotational mechanics (conceptual doubt)

• @Shwetanshu_2018 bro like this is one method another mthd is taking system like here sir told at time 4.50 after then when we can write
net {torque}=netI_{com}× \alpha

@Viram_2019 bro ur point of view to

• and in which point about the above mentioned relation is written and why
@Shiva_1 sir please u also share ur approch

why only point p is choosen i m taking the above mthd only post pic.

@Sneha_2021 Point P is taken because other points are accelerating and hence if you’ll consider other points then you’ll have to consider torque due to pseudo force also and that will simply complicate the calculation

In the text image you uploaded it’s clearly mentioned that it is considered about point P and the reason for that is explained in post #4

i got it then when to think of system net torque relation is their may be any mistake applying this concept in problem
what may be that mistake.

@Sneha_2021 I’m sorry but I didn’t get what do you mean by that , can you please clarify that in which of the above solutions you are facing difficulties? Is it the text one or the video solution ?

sir if u will see video at last sir is focussing in one point that {if i will tell u one mistake then u can apply the concept easily} i want to know what may be that mistake
or in short we can apply this
net {torque}=netI_{com}× \alpha everywhere in rotational motion question or is their any
disclaimer to use it..

This can be written ‘only about instantaneous centre of rotation’ that’s basically what i have explained that it can be written only about P (ICR) about other points it will yield wrong result due to non inertial frame .

You can apply this in any question but only about it’s ICR

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thanks bro as always u r best physics mentor..

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